1. Field of the Invention
The present invention relates to a method for depositing polycrystal Si film. More particularly, the present invention relates to a method for depositing a polycrystal Si film having excellent crystallinity and high conductivity.
2. Description of the Related Art
Conventionally, the following methods (a) and (b) have been known as examples of a method for depositing a polycrystal Si film:
(a) the thermal CVD for depositing a polycrystal Si film on a substrate by blowing a material gas, such as SiH.sub.4, to a substrate heated at a high temperature and producing deposition species by decomposing the material gas utilizing thermal energy; and PA1 (b) a combination method of the CVD and annealing for depositing a polycrystal Si film on a substrate, which method comprises a step of preparing an amorphous Si film or a polycrystal Si film having small-diameter particles on a substrate by the CVD including glow discharge plasma decomposition, a step of melting the film using laser, infrared light, or electric furnace, and a step of chilling-treating the resultant film. PA1 (i) a time-period (t.sub.1) for introducing the material gas and hydrogen gas; PA1 (ii) a time-period (t.sub.2) for introducing the doping gas and hydrogen gas; and PA1 (iii) a time-period (t.sub.3) for introducing hydrogen gas alone. Additionally, another object of the present invention is to provide a method for depositing a polycrystal Si film having the steps of: PA1 (i) a time-period (t.sub.1) for introducing the material gas and the hydrogen radicals; PA1 (ii) a time-period (t.sub.2) for introducing the doping gas and the hydrogen radicals; and PA1 (iii) a time-period (t.sub.3) for introducing the hydrogen radicals alone. PA1 generating SiF.sub.n (n=1 to 3) radicals, dopant radicals, and hydrogen radicals in separate spaces adjacent to a deposition space; PA1 introducing the radicals into the deposition space:
Since heat treatment at about 1000.degree. C. or more is required for preparing a polycrystal Si film according to the above-mentioned methods, those materials, e.g., glass, metals, which are used as a substrate in general can not be employed for polycrystal Si film deposition. Therefore, a method for depositing a polycrystal Si film at a low temperature, 500.degree. C. or less, has been in demand for using metals, glass, and the like as a substrate of the deposition.
According to a known method (c), the afore-described low temperature process is attained by decomposing a material gas by discharge or light. The plasma CVD and the photo CVD are typically used for decomposing a material gas. The Plasma CVD has the advantage of providing a faster film-deposition rate compared with the photo CVD. In general, according to those methods, it is possible to prepare a polycrystal Si film on a substrate maintained at a low temperature, such as from 300 to 450.degree. C., when a material gas, e.g., SiH.sub.4, SiF.sub.4, or Si.sub.2 H.sub.6, is diluted in a large amount of hydrogen gas and discharge electric power is increased.
However, the film prepared by the above method (c) contains a large portion of amorphous Si. Thus, satisfactory photoelectric transfer characteristic of the film is not obtained in some cases, moreover, the diameter of the crystallized particles becomes as small as 50 .ANG. or less. It is considered that the reason for these problems is such that sufficient structural relaxation is not achieved in the film, since deposition species have been produced by a non-equilibrium reaction of glow discharge plasma, poured on the substrate, and trapped into the film. Therefore, a method for depositing a polycrystal Si film has been in demand, which method is able to proceed at a low temperature and attain satisfactory structural relaxation of the resultant film.
Method (d) is an example of simultaneously achieving the afore-mentioned low-temperature process and structural relaxation. In accordance with the method (d), during decomposition procedure, material-gas supply is stopped midway or a substrate is moved to another plasma space to which no material-gas is supplied. Thereby, the film deposition is periodically stopped and the film surface is exposed to hydrogen plasma in the course of deposition. As a result, structure relaxation of the film is achieved by chemical annealing with hydrogen radicals, improving the crystallinity of the film.
However, according to the method (d), the crystallinity of an n-type or p-type polycrystal Si film decreases when the film is prepared from a material gas mixed with a doping gas, such as PH.sub.3, B.sub.2 H.sub.6, or BF.sub.3. In the case that the material gas alone is used without being mixed with a doping gas, a polycrystal Si film is produced with excellent crystallinity. Therefore, a method has been in demand for depositing an n-type or p-type polycrystal Si film with excellent crystallinity using a material gas and a doping gas.